This invention relates to gas turbine combustion technology and more particularly, to late-lean-injection fuel injector configurations.
Currently, some gas turbine engines fail to operate at high efficiency and produce undesirable air-polluting emissions. The primary air-polluting emissions usually produced by turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide and unburned hydrocarbons. To this end, since oxidation of, e.g., molecular nitrogen, in gas turbine engines is dependent upon a high temperature in the combustor and the residence time for the reactants at the high temperature within the combustor, the level of thermal NOx formation is reduced by maintaining the combustor temperature below the level at which thermal NOx is formed or by limiting the residence time for the reactants at the high temperatures such that there is insufficient time for the NOx formations to progress.
One temperature-controlling method involves premixing of fuel and air to form a lean mixture prior to combustion. However, it has been seen that for heavy duty industrial gas turbines, even with the use of premixed lean fuels, the required temperatures of the combustion products are so high that the combustor must be operated with a peak gas temperature in the reaction zone that exceeds the thermal NOx formation threshold temperature, resulting in significant NOx formation.
Late lean injection (LLI) techniques have been developed to reduce NOx formation. Specifically, the purpose of LLI is to reduce NOx formation by reducing the residence time of fuel and air within the combustor. This is achieved by injecting a portion of the fuel and air into the combustor at a location downstream of the main combustion zone. In this way, the LLI fuel and air are combusted but do not travel as far through the combustor. As such, as long as sufficient fuel and air mixing occurs, the LLI fuel and air generally do not form as much NOx as would otherwise be produced.
In the implementation of LLI, tube-in-tube injectors may be employed, as described, for example, in U.S. 2010/0170216 A1. Such injectors actively feed fuel to the interior of the transition zone between the combustor and the turbine. The injectors include a fuel injection tube extending along and through a larger diameter tube or sleeve through which air is passively fed to the transition zone. The presently configured LLI injectors, however, give rise to potential flashback problems where ignited gas in the transition zone enters the LLI injector nozzles.
There remains a need for more efficient LLI fuel injectors that produce lesser NOx and which provide greater flashback resistance.